Record of Sediment–Organism Interactions in a Siliciclastic Marginal Marine Depositional System: Implications on Late Palaeozoic Palaeogeography of Eastern Gondwanaland

Ichnoassemblages and their distribution patterns, coupled with typical sedimentary facies architecture are useful in comprehending the complex dynamic processes in brackish‐water delta‐estuary depositional environments. The middle‐Permian Barren Measures Formation, West Bokaro Basin, India, records such complex processes due to fluvio–marine interactions, as revealed by integrated sedimentological‐ichnological analyses. Thirteen lithofacies grouped into eleven lithofacies associations, showing abundant tide‐wave signatures, along with nine low‐diversity ichnoassemblages comprised of sixteen marine‐affinitive soft‐firmground ichnogenera are documented here. The stacking pattern of the facies associations and the corresponding bioturbations exhibit brackish water, fluvio–marine interactive estuary‐delta transitional depositional system. The lower part of the succession, outcropped in the southern section, depicts a systematic changeover from the central estuarine marshy overbank to outer estuarine subtidal‐intertidal sand flat‐mud flat, followed by open marine prodelta. Corresponding ichnofacies, namely, depauperate, salinity‐stressed ichnoassemblages in the central estuary to more marine‐affinitive ichnotaxa in the outer estuary depositional systems imply temporally increasing marine inundation in the retrogradational tide‐wave mixed estuary. The transition from estuary to delta is evident from the open coast intertidal sand flat‐prodelta/offshore transitional depositional systems, densely colonized by more saline, open marine ichnoassemblages. The upper part of the succession, outcropped in the northern section, records stacking of delta‐front deposits with brackish‐water suspension‐feeding ichnoassemblages, followed by subtidal mouth bar‐interbar complex with intermittent, low‐diversity, diminutive ichnotaxa and fluvial channel—tidal overbank depositional systems, with dense, brackish water ichnoassemblages in the overbank. The coarsening‐up stacking pattern portrays increasing fluvial influence over the marine processes characteristic of a progradational, mixed tide‐wave delta. Comparing the inferred estuary‐delta transitional model with contemporary Gondwana basins elucidates a refined palaeogeography of the continental Gondwana basins in eastern India, providing strong evidence for marine encroachments until mid‐Permian (post Guadulupian).